class Solution {
    public String tree2str(TreeNode root) {
        if(root == null) {
            return null;
        }
        StringBuilder stringBuilder = new StringBuilder();
        tree2strChild(root,stringBuilder);
        return stringBuilder.toString();
    }
    public void tree2strChild(TreeNode t,StringBuilder stringBuilder) {
        if(t == null) {
            return;
        }

        stringBuilder.append(t.val);
        if(t.left != null) {
            stringBuilder.append("(");
            tree2strChild(t.left,stringBuilder);
            stringBuilder.append(")");
        }else {
            //左边为空了
            if(t.right != null) {
                //右边不为空
                stringBuilder.append("()");
            }else {
                //右边为空
                return ;
            }
        }

        if(t.right == null) {
            return;
        }else {
            stringBuilder.append("(");
            tree2strChild(t.right,stringBuilder);
            stringBuilder.append(")");
        }
    }
}